Low-dose spiral CAT scans are already part of the arsenal to try to detected early-stage lung cancer among heavy smokers who are most at risk.

Problem

The problem, though, is that these scans also pick up benign lesions in the lungs - a problem of "false positives" that make it difficult to use this method as a large-scale screening programme.

The cancers were caught at the early stages. Twenty-one out of the 22 were removed through surgery, a success rate of 95 percent.

Selective use of PET, though, helps to increase the accuracy by focussing on specific lesions that seem quietly cancerous or heading that way.

The pioneers of the technique, Ugo Pastorino from the National Cancer Institute in Milan, Italy and Peter Boyle of the European Institute of Oncology, recruited 1,035 heavy smokers who had smoked at least 26 cigarettes per day for 37 years.

Their scanning programme picked up 11 cancers in the first scan, and another 11 when a patient was scanned a year later to follow up on specific lesions that measured up to five millimetres.

The cancers were caught at the early stages. Twenty-one out of the 22 were removed through surgery, a success rate of 95 percent. There was one death from lung cancer.

CAT scans use X-rays to obtain images of the body at different angles. The data is then reconstructed by computer to gain a cross-section of tissues and organs.

Spiral CAT scans is a term describing the spiral path taken by the X-ray tube as it rotates continually around the patient.

PET scanning entails giving the patient a glucose solution or other drink, to which a mildly radioactive substance has been added.

A gamma-ray detector then follows the tagged glucose as it is carried in the bloodstream and sees how it is absorbed by the body's tissue.

Lung cancer, overwhelmingly caused by smoking, claims 1.3 million lives a year around the world, a higher toll than any other form of cancer.